The present invention relates to a wavelength dispersion measuring apparatus and polarization dispersion measuring apparatus and, more particularly, to a wavelength dispersion measuring apparatus for measuring wavelength dispersion occurring when light passes through an object to be measured such as an optical fiber and a polarization dispersion measuring apparatus for measuring polarization dispersion occurring when light passes through an object to be measured.
For example, the speed at which an optical signal propagates in an optical fiber changes in accordance with the wavelength of the optical signal.
Accordingly, the pulse width (time width) of the pulse waveform in an output optical pulse signal from a light source having a wavelength spread extends in an optical fiber.
Since the transmission band of an optical fiber is inversely proportional to pulse width, this transmission band has influence on limitations on the transmission speed of an optical signal.
Hence, measuring the transmission speed (wavelength dispersion) in an optical fiber for each wavelength is a crucial performance testing item for the optical fiber.
In particular, a very high-speed optical signal faster than 100 Gbit/s, which is to be used in a next-generation large-capacity optical network, has a narrow optical signal pulse width of a few ps, i.e., has a large wavelength spread. Therefore, the wavelength dispersion of an optical fiber has large influence on optical transmission.
Also, in the pulse generation technologies the wavelength dispersion of an optical fiber has large influence on the generation ratio of high-quality pulses, i.e., transform-limited optical pulses. So, the wavelength dispersion measurement is becoming a more important item.
As methods of measuring the wavelength dispersion, (a) time resolved spectroscopy, (b) pulse method, (c) interference method, (d) differential method, (e) phase difference method, and the like have been proposed.
Of these methods (a) to (e), a pulse method proposed in Jpn. Pat. Appln. KOKAI Publication No. 6-174592 and comparatively frequently practiced will be described below with reference to FIG. 13.
The wavelength of an output white pulse with a broad wavelength range coming from a white pulse light source 1 is limited to a specific wavelength by a variable wavelength optical band-pass filter 2. After that, an optical demultiplexer 3 demultiplexes this white pulse into an incoming optical pulse 4 and a reference optical pulse 5.
The incident optical pulse 4 enters a fiber 6 to be measured and then enters one input of an optical multiplexer 7 through the fiber 6.
The reference optical pulse 5 directly enters the other input of the optical multiplexer 7.
The optical multiplexer 7 multiplexes the incident optical pulse and the reference optical pulse 5 and inputs a multiplexed optical signal 8 to a delay time detecting means 9.
From this multiplexed optical signal 8, the delay time detecting means 9 calculates a delay time t.sub.D of the incident optical pulse with respect to the reference optical pulse 5.
That is, the incident optical pulse 4 produces a time delay by passing through the fiber 6. Therefore, multiplexing this incident optical pulse 4 and the reference optical pulse 5 having no time delay produces two peaks in the signal waveform of the multiplexed optical signal 8.
The time difference between these two peaks is the delay time t.sub.D.
While, therefore, a wavelength .lambda. of light passing through the tunable wavelength optical band-pass filter 2 is changed, a delay time t.sub.D (.lambda.) at each wavelength .lambda. is measured.
The wavelength dependence of this delay time t.sub.D (.lambda.) is the wavelength dispersion characteristic.
Next, a pulse method proposed in Jpn. Pat. Appln. KOKAI Publication No. 4-177141 will be described below with reference to FIG. 14.
An output optical pulse from an ultrashort pulse generator 11 passes through an optical fiber 12 to be measured and is demultiplexed into two optical pulses A and B by an optical demultiplexer 13.
A tunable wavelength band-pass filter 14 passes only a specific wavelength of the optical pulse A to form a first optical pulse.
The optical pulse B passes through a delay line 15 to form a second optical pulse.
An optical multiplexer 16 multiplexes the first and second optical pulses, and a photodetector 17 converts the multiplexed pulse into an electrical signal. A pulse waveform measuring device 18 measures the relative delay time difference between the first and second optical pulses as a function of wavelength. Consequently, the wavelength dependence of delay time described above is obtained.
Another important characteristic of an optical communication medium such as an optical fiber is the polarization dispersion characteristic.
That is, in an optical fiber having an ideal truly circular sectional shape, the transmission speed of an optical pulse signal propagating in this optical fiber does not change regardless of the direction of this optical pulse signal in a fiber section.
If, however, the sectional shape of optical fiber is an ellipse rather than a true circle or the optical fiber is bent to partially flatten the sectional shape, the transmission speed of an optical pulse signal propagating in this optical fiber changes in accordance with the polarization direction.
Hence, measuring any difference in transmission speed between optical signals propagating in different polarization directions in an optical fiber, i.e., measuring the transmission speed (polarization dispersion) in an optical fiber for each polarization direction is also a crucial performance testing item for the optical fiber.
This polarization dispersion can also be obtained by measuring a group delay amount for each pair of polarization components perpendicular to each other by this pulse method.
Unfortunately, the above measuring methods still have the following problems to be solved.
That is, in the wavelength dispersion measuring method shown in FIG. 14, an optical pulse passing through the optical fiber 12 is demultiplexed into two optical pulses, and a delay time is measured at each wavelength by using one optical pulse as reference light on the time base. Accordingly, the method is unaffected by a change in the optical path length of the optical fiber resulting from an external factor such as a temperature change.
The tunable wavelength band-pass filter 14, however, picks up a specific wavelength from the output light from the ultrashort pulse light source 11. Therefore, no present technology can prevent an increase in the pulse width (time width) of the optical pulse passing through the filter 14 resulting from limitations on the frequency band.
Accordingly, this method cannot easily locate the pulse peak position and is prone to many measurement errors.
For example, assuming that the passing wavelength width is 0.1 nm, the pulse width (time width) of the extracted optical pulse is probably 20 ps (picoseconds) or more.
As the delay time difference measuring means, the pulse waveform measuring device 18 constructed of, e.g., an electrical sampling oscilloscope is used.
Hence, the measuring method shown in FIG. 14 is effective to measure the dispersion of a long optical fiber (a few km or more) but difficult to use to measure low dispersion of a short optical fiber such as an EDF (the basic length of fiber amplifier EDF) about 20 m long.
In the wavelength dispersion measuring method shown in FIG. 13, the white pulse light source 1 for outputting a short pulse light group over a continuous broad wavelength range to accurately measure the wavelength dispersion is combined with the delay time difference measuring means 9 constructed of, e.g., a streak camera or the like. This enables more accurate measurement than in the wavelength dispersion measuring method shown in FIG. 14.
That is, since the spectral width of the white pulse light source 11 is as large as 200 nm, an optical band-pass filter 2 having a band width of about 1 nm can be inserted. Consequently, an optical pulse having a pulse width (time width) of a few ps can be obtained with no problem, so the peak position can be easily located.
When a streak camera is used as the delay time detecting means 9, however, the accuracy of in the time domain is 0.3 ps or more. This makes this method unsatisfactory to measure the wavelength dispersion of a low-dispersion object to be measured such as a short optical fiber.
This similarly applies to polarization dispersion measurement.